The manufacture of integrated circuits generally involves an elaborate system of fabricating semiconductor devices on a substrate and connecting the devices together. The devices are connected by a process generally referred to as metalization, in which connecting lines of metal, often aluminum, are applied by vacuum deposition or other suitable process.
The performance level of semiconductor devices employing a conventional single metal layer connecting the devices is becoming inadequate. Modern, high performance devices utilize multilevel metal interconnections. Multilevel connections may be constructed by depositing a dielectric or insulating layer over a first metal layer, etching via holes throughout the dielectric layer, and then depositing a second metal layer which fills the via holes to connect with the first metal layer. These devices offer higher device density and shortened interconnection lengths between the devices.
Since each of these metal and dielectric layers has an appreciable thickness, the substrate is left with non-planar topography as the various layers are patterned on top of one another. This type of non-planarity is often unacceptable in high density devices because the depth of field of the lithographic equipment that is used to print the smaller line width circuits on the substrate does not have a depth of focus sufficient to compensate for even small variations in substrate planarity.
In addition to the non-planarity caused by the fabricated device patterns, in-process substrate polishing, or planarization, must account for variations in overall substrate flatness as well. During the fabrication process, for example, the substrates may become bowed or warped.
In-process substrate polishing equipment, therefore, requires the specialized ability to achieve global, uniformly planar substrate surfaces in spite of these topographical substrate defects and variations. Chemical-mechanical polishing has gained wide acceptance as an effective means of achieving the global substrate surface planarity required by advanced devices employing multilayer metalization.
FIG. 1 shows a cross section of a typical prior art chemical-mechanical polishing arrangement. A typical device includes a substrate carrier having a generally circular pressure plate or platen 1 that supports a single substrate 3. Often a carrier film 2 is interposed between the platen 1 and the substrate 3 to partially accommodate substrate thickness variations. The substrate carrier is equipped with means to provide a downward force, urging the substrate 3 against an abrasive pad or strip 5, onto which is fed an abrasive polishing fluid 7. A containment ring 4 generally surrounds the substrate to prevent it from slipping off the platen during polishing. Movement of the substrate relative to the pad, in the presence of the polishing fluid and under pressure from the substrate carrier, imparts a combination of chemical and mechanical forces to the substrate 3, the net effect of which is global planarization of the substrate surface.
A closer look at the chemical-mechanical polishing process reveals that perfected global planarization of a substrate surface is achieved only when there is a uniform removal rate, at every point on the substrate surface. For any given point on the surface of the substrate, the polishing process can be described by Preston's equation: EQU R=K.sub.p .times.P.times.V,
where R is the removal rate; K.sub.p is a function of consumables (abrasive pad roughness and elasticity, surface chemistry and abrasion effects, and contact area), P is the applied pressure between the substrate and the abrasive pad; and V is the relative velocity between the substrate and the abrasive pad.
To obtain optimum results, each of these variables must be held perfectly constant or the effects of their variability must be accommodated for in corrective elements elsewhere in the system. In this regard, the containment ring plays a critical role in controlling the variables which determine the removal rate during polishing.
The constant K.sub.p in Preston's equation, is intended to reflect certain properties relating to the abrasive pad or strip. But many of these properties are not constant, and instead vary to the detriment of optimum planarization. For example the abrasive pad may have localized areas of varying roughness, varying frictional coefficients, varying elasticity (both with respect to compression as the pad is pressed against it but with respect to the amount of stretch encountered in the plane of polish as the pad is pushed across it), and varying amounts of polishing fluid present.
With regard to the applied pressure between the substrate and the abrasive pad, there are many factors that tend to contribute to the formation of localized regions of varying pressure, or pressure gradients, at different locations on the substrate surface. Such contributory factors might include angular misalignment of the substrate to the abrasive pad, frictional forces created by the polishing action that tend to urge the substrate into angular misalignment relative to the abrasive pad, thickness variations in the substrate itself or an uneven platen, and dynamic phenomena resulting from the movement of the substrate against the pad.
One example of such dynamic phenomena is the dynamic wave that is often formed in the abrasive pad at the trailing edge of the substrate as it is polished. Typically, this is evidenced by rings of over polish then under polish on the substrate surface; the substrate being impacted the most at the perimeter and decreasing toward the center as the wave in the pad was dampened out. Often the area of non-uniform polishing around the outer periphery of a substrate is not useable for device formation and is referred to as the edge exclusion zone. To maximize the useful area of a processed substrate, it is highly desirable to minimize the edge exclusion zone.
Devices known in the art have attempted to control some of the troublesome variables that cause non-uniform removal rates during polishing. For example, it is common for the substrate carrier of a device suited for chemical-mechanical polishing, to include some means for automatically aligning the platen and substrate surface with the surface of the polishing pad or strip. Some devices accommodate substrate misalignment by adding a layer of flexible material between the platen and the substrate. The flexible material deforms to allow the angularity of the substrate to match that of the abrasive pad, and at the same time provides support of the substrate during the polishing process. While this somewhat alleviates the pressure gradients that result from substrate misalignment, it does not effectively eliminate them. Where the pad has deformed to accommodate any substrate misalignment, it has stored energy according to Hooke's law of elasticity, and therefor reacts with a higher force localized to the area of deformation.
Other devices endeavor to ensure proper alignment of the substrate to the abrasive head by providing a mounting structure for the substrate carrier that is capable of allowing the substrate carrier and platen to pivot to varying angles of tilt while simultaneously transmitting the downward force required for polishing. One such device is found in U.S. Pat. No. 4,270,314 to Cesna which discloses a mounting structure between the pressure plate and the supporting shaft that includes a bearing means suited both for allowing free rotation of the pressure plate relative to the supporting shaft (at varying angles of tilt) and for transmitting axial loads. Another such device is disclosed in U.S. Pat. No. 5,377,451 to Leoni et al. In Leoni '451 a pressure plate is connected by a universal joint assembly which permits rotation about the pressure plate axis and permits universal pivoting motion about a pivot point on the pressure plate axis.
In the static mode, such devices as described above appear to allow the substrate surface to align with the abrasive pad or strip as pressure is applied by the substrate carrier. But during polishing misalignment may again occur as a result of the frictional forces involved. For example, in a self-aligning substrate carrier having a pivot point at a distance above the polishing surface, an unwanted moment is created as a result of the frictional force generated in the plane of the polishing surface during polishing. This moment disadvantageously causes the platen and substrate to rotate about the pivot point, thus forcing the leading edge more severely into the abrasive pad. As a result the leading edge experiences over polish, and if the substrate carrier is selected to traverse a polishing path over the abrasive pad that involves travel in a variety of directions, a ring of over polish will be formed around the perimeter of the substrate.
Such removal rate problems are often compounded by the interaction of the various factors that arise during polishing. For example, as the friction moment causes the leading edge to dig in and over polish, as just discussed, the non-linear frictional forces on the pad may induce dynamic waves in the pad material thus causing regions of differing pressure. At the same time, the polishing fluid may not be evenly distributed under the substrate thus causing some areas to have higher or lower removal rates.
Under these complex dynamics involved in the polishing of a substrate, the function and relation of the containment ring becomes very important. At a minimum, the containment ring must provide lateral support at the edge of the substrate to prevent the substrate from slipping out from underneath the substrate carrier during polishing. To varying degrees, a number of known devices have improved over the basic containment ring shown in FIG. 1.
One such containment ring may be found in U.S. Pat. No. 5,398,459 to Okumura et al. Okumura et al. generally discloses the use of a rigidly mounted containment ring that is larger than the diameter of the substrate in a polishing apparatus involving a rotating abrasive pad as well as a rotating substrate carrier. In Okumura et al., the rotation of the turntable (to which the abrasive pad is affixed) imparts a pressing force in a direction parallel to the upper surface of the turntable to the workpiece so that an outer periphery of the workpiece contacts an inner periphery of the retaining ring, and the rotation of the retaining ring imparts a rotational force to the workpiece so that the workpiece undergoes a planetary motion relative to the platen.
Another containment ring known in the art may be found in U.S. Pat. No. 5,423,558 to Koeth et al. which discloses a containment ring rigidly mounted to the platen or carrier. To reduce unwanted edge effects, Koeth et al. discloses the use of a gimbaling head and a perimeter cavity cut into the platen just inside of the containment ring apparently to alleviate polishing defects at the perimeter by allowing the substrate perimeter to deflect.
Other containment ring devices employ the ring to have an effect on the abrasive pad in front of the leading edge of the substrate. Such a containment ring is disclosed in U.S. Pat. No. 4,954,142 to Carr et al. The containment ring disclosed in Carr et al. involves a containment ring in the form of a pressure plate that is spring mounted to the platen base. The containment ring in Carr et al. functions to depress the abrasive pad around the substrate during polishing and consequently lessening the pressure exerted against the edges of the substrate by the abrasive pad.
A similar device may be found in U.S. Pat. No. 5,498,199 to Karlsrud et al. where there is disclosed a ring that has an adjustable height relative to the platen. The containment ring is rigidly threaded onto the platen and adjustment requires turning the ring by hand. By such adjustment the amount of abrasive pad depression may be controlled, but there is no provision for ring pressure independent of the platen, nor does it possess any structure capable of performing self-correction for unwanted frictional moments.
Still another type of containment ring device may be found in U.S. Pat. No. 5,335,453 to Baldy et al. Baldy et al. discloses a spring mounted containment ring having a break, in which the entire outer perimeter of the substrate is supported. The ring is not designed to contact the polishing pad material.
From the preceding discussion, it should be apparent that it would be desirable to have a containment ring that provides for a uniform removal rate during polishing. It would be desirable to have a containment ring that has the ability to self-correct for the effects of unwanted friction moments and other polishing errors. In addition, it would be highly desirable to have a containment ring that exerts a pressure independent of the platen and that is adjustable during the polishing operation. It would also be desirable to have a containment ring that allows polishing fluid to pass to the substrate.